Radio waves are electromagnetic waves that can be used in wireless communication. The frequencies of these waves serve as physical communication channels. The radio frequency (RF) spectrum has a finite range of frequencies, and thus a limited number of channels. In the United States, for example, the Federal Communications Commission (FCC) decides how the spectrum is allocated and what bands are used for what purpose.
Communication signals on the same channel interfere, assuming the strengths of the signals are non-negligible due to transmission power and distance. Also, communication signals on adjacent channels may interfere with communications on the desired channel because of inadequate filtering, tuning or frequency control. Adjacent channel interference can increase with an increase in signal power in adjacent channels.
Most countries of the world have allocated certain frequency spectrums for commercial use as “unlicensed” wireless bands. For example, the FCC has designated license-free bandwidth segments for industrial, scientific and medical (ISM) uses. Various commercial applications use this unlicensed bandwidth for short range wireless communication.
Channels are not allocated within the license-free band. Commercial devices designed to operate in the license-free band are constrained to transmit using a relatively low power, which allows more commercial devices to use the unlicensed frequency bands by increasing the reuse of a frequency. Spread spectrum systems mitigate interference by spreading their information over a much larger bandwidth than the information requires. This has the advantage of spreading any narrowband interference encountered within the channel over a large bandwidth which can then be integrated out by the receiver. The transmitter and receiver coordinate and manage the spreading sequences. This adds complexity and power to spread the signal using either fast frequency hopping or direct sequence phase manipulation. This added complexity and power may prevent these schemes from being used in ultra-low power communications systems.